1. Field of the Invention
The present invention relates to drive circuitry for driving a liquid crystal display panel (LCD panel) by applying a common voltage and a segment voltage to the LCD panel. The invention also pertains to a liquid crystal display apparatus provided with the above type of drive circuitry.
2. Description of the Related Art
FIG. 8 is a block diagram illustrating an example of conventional liquid crystal display apparatuses. In FIG. 8, voltages V.sub.1 through V.sub.6 generated by a bias power supply circuit 1 are inverted at a timing provided by an alternating-current (AC) signal circuit 2, and are applied to common electrodes and segment electrodes of an LCD panel 3.
FIG. 9 is a circuit diagram illustrating an example of the configuration of the bias power supply circuit 1. In FIG. 9, the resistances of the respective resistors R.sub.1 through R.sub.5 are determined, for example, as follows: the resistances of the resistors R.sub.1, R.sub.2, R.sub.4, and R.sub.5 are 1 [k.OMEGA.], while the resistance of the resistor R.sub.3 is 11 [k.OMEGA.]. An amplifier 4 is provided to maintain the voltages V.sub.3 through V.sub.6 in the case of the generation of overcurrent. FIG. 9 shows that the bias power supply circuit 1 divides a power voltage V.sub.EE (for example, 30 [V]) supplied from an external source, such as a personal computer, so as to generate the voltages V.sub.1 through V.sub.6.
The voltages V.sub.1 through V.sub.6 shown in FIG. 9 can be calculated as follows. EQU V.sub.1 =30 [V] EQU V.sub.6 =(30/15).times.14=28 [V] EQU V.sub.3 =(30/15).times.13=26 [V] EQU V.sub.4 =(30/15).times.2=4 [V] EQU V.sub.5 =(30/15).times.1=2 [V] EQU V.sub.2 =0 [V]
The intermediate voltage (15[V]) between the power supply voltage V.sub.EE (30[V]) and the ground voltage (0[V]) is determined to be the center voltage. Then, the voltages V.sub.1 and V.sub.2, V.sub.3 and V.sub.4, and V.sub.5 and V.sub.6 are respectively symmetrical to each other relative to the center voltage. In other words, the voltage obtained by inverting the voltage V.sub.1 is V.sub.2, and similarly, the inverted voltages of V.sub.3 and V.sub.5 are V.sub.4 and V.sub.6, respectively.
FIG. 10 illustrates a wiring pattern of the common electrodes and the segment electrodes of the LCD panel 3. FIG. 10 represents that the LCD panel 3 is formed by wiring, for example, 480 common electrodes and 640 segment electrodes, in an orthogonal direction. Each intersection between a common electrode and a segment electrode designates one pixel of the LCD panel 3. With this arrangement, a voltage equal to an amount of (a voltage applied to a common electrode)--(a voltage applied to a segment electrode) is applied to a liquid crystal layer of each pixel.
The voltage applied to a common electrode (hereinafter referred to as "the common voltage") determines whether or not the pixels on the common electrode are to be selected, i.e., whether the pixels are in the "selective state" or the "non-selective state". In contrast, the voltage applied to a segment electrode (hereinafter referred to as "the segment voltage") determines the display status, i.e., the "on-state" or "off-state", of the pixels on the selected segment electrode.
More specifically, in order to switch on a certain pixel, as shown in FIG. 10, the voltage V.sub.1 is applied to the corresponding common electrode so as to select the pixel, and also, the voltage V.sub.2 is applied to the corresponding segment electrode. Accordingly, the voltage (V.sub.1 -V.sub.2 =30 [V]) is applied to the liquid crystal layer of the pixel positioned at the intersection between the designated common electrode and the segment electrode. The selected pixel is thus set to the on-state.
Conversely, in order to switch off a certain pixel, as illustrated in FIG. 10, the voltage V.sub.1 is applied to the associated common electrode so as to select the pixels, while the voltage V.sub.4 is applied to the corresponding segment electrode. Thus, the voltage (V.sub.1 -V.sub.4 =26 [V]) is applied to the liquid crystal layer of the pixel located at the intersection between the specified common electrode and the segment electrode. As a result, the selected pixel is set to the off-state. In the above operation, while the voltage V.sub.1 is applied to the corresponding common electrode, the voltage V.sub.5 is applied to the other common electrodes in order to render the pixels other than the selected pixel in the off-state.
FIG. 11 illustrates the waveforms of the common voltage and the segment voltage applied to a selected pixel during one frame. The term "one frame" indicates a frame during which all of the pixels forming one frame of a liquid crystal display apparatus are displayed either in the on-state or the off-state. Moreover, "one frame" consists of a duration during which a selected pixel is set to the on-state or the off-state (i.e., the selective duration) and a duration during which the other pixels are placed to the on-state or the off-state (i.e., non-selective duration). In FIG. 10, the horizontal axis represents time; it will now be assumed that the time axes of the upper and lower waveforms (the common voltage waveform and the segment voltage waveform) in FIG. 10 are consistent.
FIG. 11 reveals that the common voltage waveform alternates during the non-selective duration between an interval at which the voltage V.sub.5 is applied (the interval A) and an interval during which the voltage V.sub.6 is applied (the interval B). During the selective duration, the common voltage waveform exhibits a voltage to be applied to the common electrode corresponding to the selected pixel. In the example shown in FIG. 11, the voltage V.sub.1 is applied during the interval A, while the voltage V.sub.2 is applied during the interval B.
The segment voltage waveform shown in FIG. 11 alternates during one frame between an interval at which a negative voltage is used (the interval A), and an interval at which a positive voltage is used (the interval B). During the interval A, the pixels are switched on with the voltage V.sub.2, while the pixels are switched off with the voltage V.sub.4. In contrast, during the interval B, the pixels are switched on with the voltage V.sub.1, while the pixels are switched off with the voltage V.sub.3.
In this manner, the common voltage waveform and the segment voltage waveform are inverted at a fixed cycle (the interval A and the interval B) in order to preserve the quality of the liquid crystal layers of the pixels. Namely, the liquid crystal layers have the property of easily deteriorating if a voltage of the same polarity is continuously applied thereto. It is thus necessary that the polarities of the voltages applied to the liquid crystal layers be inverted at a fixed cycle (between the interval A and the interval B).
The signal having a fixed cycle is an AC signal (DF') output from the AC signal circuit 2 shown in FIG. 8. Namely, the AC signal circuit 2 generates an AC signal (DF'), which is a rectangular wave signal inverting at a fixed cycle. Then, common drivers 5 and segment drivers 6 determine, based on the AC signal (DF'), the cycle of the common voltage and the segment voltage, i.e., the interval A or the interval B indicated in FIG. 11. In the liquid crystal display apparatus shown in FIG. 8, since the shared AC signal (DF') is supplied to the common drivers 5 and the segment drivers 6, the common voltage waveform and the segment voltage waveform are inverted at the same timing, as illustrated in FIG. 11.
FIG. 12 illustrates the waveform of the voltage applied to the liquid crystal layer of a selected pixel during one frame period. This waveform is obtained when the common voltage and the segment voltage indicated in FIG. 11 are applied to the selected pixel. FIG. 12 reveals that 2 [V] or -2 [V] is applied to the liquid crystal layer of the selected pixel during the non-selective duration.
As shown in FIG. 11, the voltage V.sub.1 (or the voltage V.sub.2) is applied to the common electrode during the selective duration, so that the selected pixel is displayed with a differential voltage between the common voltage and the segment voltage. In the example shown in FIG. 12, the designated pixel is displayed in the off-state with the voltage (V.sub.1 -V.sub.4 =26 [V]). If, however, it is desired that the designated pixel is switched on, the voltage (V.sub.1 -V.sub.2 =30 [V]) is applied thereto.
The following problems are, however, encountered by the above-described known liquid crystal display apparatus. When a narrow black bar is displayed in a white background on the screen of the LCD panel, a white bar having a brightness level higher than the white background (hereinafter referred to as "white crosstalk") disadvantageously appears on a line extending from the black bar. Moreover, when a thick black bar is displayed in a white background on the screen, a black bar having a brightness level slightly lower than the white background (hereinafter referred to as "black crosstalk") unfavorably emerges on a line extending from the black bar.